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Python applications will often use packages and modules that don’t
come as part of the standard library. Applications will sometimes
need a specific version of a library, because the application may
require that a particular bug has been fixed or the application may be
written using an obsolete version of the library’s interface.

This means it may not be possible for one Python installation to meet
the requirements of every application. If application A needs version
1.0 of a particular module but application B needs version 2.0, then
the requirements are in conflict and installing either version 1.0 or 2.0
will leave one application unable to run.

The solution for this problem is to create a virtual environment, a
self-contained directory tree that contains a Python installation for a
particular version of Python, plus a number of additional packages.

Different applications can then use different virtual environments.
To resolve the earlier example of conflicting requirements,
application A can have its own virtual environment with version 1.0
installed while application B has another virtual environment with version 2.0.
If application B requires a library be upgraded to version 3.0, this will
not affect application A’s environment.

The module used to create and manage virtual environments is called
venv. venv will usually install the most recent version of
Python that you have available. If you have multiple versions of Python on your
system, you can select a specific Python version by running python3 or
whichever version you want.

To create a virtual environment, decide upon a directory where you want to
place it, and run the venv module as a script with the directory path:

python3-mvenvtutorial-env

This will create the tutorial-env directory if it doesn’t exist,
and also create directories inside it containing a copy of the Python
interpreter, the standard library, and various supporting files.

Once you’ve created a virtual environment, you may activate it.

On Windows, run:

tutorial-env\Scripts\activate.bat

On Unix or MacOS, run:

sourcetutorial-env/bin/activate

(This script is written for the bash shell. If you use the
csh or fish shells, there are alternate
activate.csh and activate.fish scripts you should use
instead.)

Activating the virtual environment will change your shell’s prompt to show what
virtual environment you’re using, and modify the environment so that running
python will get you that particular version and installation of Python.
For example:

You can install, upgrade, and remove packages using a program called
pip. By default pip will install packages from the Python
Package Index, <https://pypi.org>. You can browse the Python
Package Index by going to it in your web browser, or you can use pip’s
limited search feature:

If you re-run this command, pip will notice that the requested
version is already installed and do nothing. You can supply a
different version number to get that version, or you can run pipinstall--upgrade to upgrade the package to the latest version: